Evaluation and grading of rat liver foci in carcinogenicity tests.

The significance of rat liver foci in carcinogenesis testing received major attention following their description in the National Cancer Institute workshop of 1974. However, the biological nature of foci remains uncertain despite numerous studies during the past 15 years. That is, in part, because criteria to define foci have been inconsistent, and studies of spontaneous foci in rats have been very few. This symposium has demonstrated that the induction or enhancement of rat liver foci, per se, may not provide sufficient evidence to classify a test compound as a carcinogen. Evidence presented suggests that some types of rat liver foci may not be related to carcinogenesis, and there is, consequently, a basis to further subclassify these lesions. In cases where foci are considered to be a part of the neoplastic process, semi-quantitative grading of these lesions, as well as of tumors, according to the extent of their development or progression, may assist in the interpretation of equivocal carcinogenicity test findings.